1. Technical Field
This invention relates to a highly tough, ERW steel pipe having a high sour resistance, and more particularly to an ERW steel pipe (which means an electric-resistance-welded pipe or tube) having a high cracking resistance even under an environment containing wet hydrogen sulfide, for example, in drilling for or transportation of petroleum and natural gas, and also having a distinguished low temperature toughness.
2. Description of the Prior Art
The recently produced petroleum and natural gas very often contain hydrogen sulfide, and in the presence of sea water, fresh water, etc. at the same time corrosition takes place not only on the steel pipe surface to reduce the pipe wall thickness, but also the hydrogen generated on the steel pipe surface due to the corrosion may diffuse into the steel body to break the steel pipe. This has been a problem. The breakage is different from the stress cracking by sulfides which has been observed in high tensile steel since early times, from the viewpoint that the breakage has been observed to occur without any additional stress from the outside.
The hydrogen diffused from the environment circumulates at the boundaries between the steel matrix and inclusions such as MnS existing in the matrix and extended long in the rolling direction, and it is gasified in the steel matrix and the breakage occurs due to the resulting elevated hydrogen gas pressure. Inclusions such as MnS act as sharp notches, and these develop into cracks in parallel to the plate surface as crack nuclei. The cracks in parallel to the plate surface are connected to one another in the plate thickness direction. This kind of cracks will be hereinafter referred to as "hydrogen induced cracking".
Various researches have been so far made on steel of high resistance to the hydrogen induced cracking, and various kinds of steel have been proposed, typical of which are utilizations of crack prevention by addition of Cu or Co., reduction in MnS by reducing sulfer content, fixation of S by addition earth elements, etc., as disclosed, for example, in Japanese Patent Publication No. 57-17065, Japanese Patent Publication No. 57-16184, etc. By virtue of these techniques, steel capable of withstanding a considerably severe environment has been made available up to now.
A seam welded steel pipe is produced by forming a steel plate such as hot coil, etc. and seam welding the shaped steel plate at the edge parts, and its essential difference from a steel plate is, needless to say, in the presence of a welded part and heat affected zone. There have been substantially no examples of investigation of the sour resistance at the weld and heat affected zone, because in the ordinary process inclusions such as MnS extending in the rolling direction exist to a large extent at the inverse V segregation parts in the case of large size ingots and at the center segregation parts in the case of continuously cast slab, and hardly exist at the edge parts of steel plates, and thus it has been common knowledge that the so called single hoop pipe produced by welding a steel plate at the edge parts to join one edge part with another has a good sour resistance at the weld and heat affected zone.
On the other hand, in the case of the so called coil-splitted ERW pipe production by dividing a hot coil into at least two strips in the width direction, and by electric-resistance-welding the strips, parts highly sensitive to a hydrogen induced cracking such as inverse V segregation parts and center segregation parts locate at one side or both sides of the weld and thus there has been the recognition of the hydrogen induced cracking. However, in this case, the same steps as those for the matrix such as reduction in inclusions such as MnS and the microsegregation of Mn, P and the like have also been taken in principle so far.
As an extensive study of the sour resistance at the weld of an ERW steel pipe, one of the present inventors found and disclosed in Japanese Patent Application Kokai (Laid-Open) No. 61-124554 corresponding to U.S. patent application Ser. No. 799978 filed on Nov. 20, 1985 that, even when there are no inclusions such as MnS extended long in the rolling direction, hydrogen induced cracking sometimes occur at the weld, and the hydrogen induced cracking occurs as cracks perpendicular to the plate surface at the weld, which differs from the cracks occuring in the base metal. Furthermore, one of the present inventors found that the hydrogen induced cracking occurs even at the single hoop pipe of essentially less microsegregation at the edge parts of a steel plate. These cracks had never been disclosed and are a problem as important as or more important than those of the hydrogen induced cracking in parallel to the plate surface of the base material. Furthermore, it was found that this cracking occurs even at the ERW pipes of steel for which the conventional steel making process is not be prevented by the conventional techniques.
Tbe production areas of petroleum and natural gas have a been recently extended over to extremely cold areas such as Alaska, USSR, and the Arctic Ocean, and line pipes for use in these areas require a distinguished low temperature toughness in the base material and also at the weld. When the fluid products contain hydrogen sulfide, it is needless to say that a sour resistance is required in addition to the low temperature toughness.
In the ERW steel pipe, the toughness is lower at the welded part than in the base material, and various researches have been so far made on the production of ERW steel pipes having a distinguished toughness even at the weld. Various methods and steel pipes have been thus far proposed, typical of which are utilizations of tough materials obtained by controlling the finishing temperature and the coiling temperature in the hot rolling process, control of grain size by limitation of the cooling speed in a seam weld heat treatment, reduction of N in solid solution, refining the grain size by the addition of Nb or V, etc., as disclosed, for example, in Japanese Patent Application Kokai (Laid-Open) No. 54-136512, Japanese Patent Application Kokai (Laid-Open) No. 57-140823, Japanese Patent Publication No. 58-53707, Japanese Patent Publication No. 58-53708, etc. ERW steel pipes having a considerably distinguished toughness have been made available up to now according to these techniques.
However, these ERW steel pipes are destined to use in a noncorrosive environment, and their use in the so-called sour environment containing hydrogen sulfide or water has not been taken into account.
As a result of extensive studies also on the toughness of the welds of ERW steel pipes, one of the present inventors found that the toughness sour-resistant ERW steel pipes is considerably more deteriorated at the welds than in the base material, and that this problem could not be solved according to any of the foregoing prior art techniques.
As a result of further studies to develop a steel pipe having a high toughness and high resistance to quite a new type of hydrogen induced cracking, that is, a cracking perpendicular to the plate surface, Miyasaka found (see Japanese Patent Application Kokai No. 62-170458) that a cause for the hydrogen induced cracking and the reduction in toughness at the weld of an ERW steel pipe 1 schematically shown in FIG. 1 is flattened inclusions of oxides existing at a weld 2 and welding heat affected zone 3 on both sides Z.sub.1 and Z.sub.2 thereof within a distance of 500 .mu.m each from the weld 2.
Furthermore, he found that, among these flattened inclusions of oxides, those inclusions whose shapes, as viewed in the cross-section within the distance of Z.sub.1 =Z.sub.2 =500 .mu.m at both sides of the weld 2 shown in FIG. 1, have a ratio of more than 2 between the length in the through thickness direction and the length in the circumferential direction and whose major axis is 2 .mu.m or longer, act as nuclei for the generation of hydrogen induced cracking, and when those inclusions whose shapes have a ratio of more than 2 between the length in the through thickness direction and the length in the circumferential direction and when 5 or more inclusions of the oxides having a major axis of 2 .mu.m or longer are included in the cross-section of 1 mm.sup.2, the hydrogen induced cracks generated as nuclei join one another and develop into macroscopic cracks.
According to further studies made by one of the present inventors, it was found that these flattened inclusions of oxides are complex oxides composed of Ca and Al as the main components, and that the inclusions of oxides existing in the base material advance in a nearly spherical shape are heated nearly up to the melting point of the steel during the seam welding, compressed from both sides by the squeeze rolls and thus deformed into a flattened form.
On the basis of the foregoing findings, one of the present inventors proposed a steel for production of ERW steel pipe having a distinguished sour resistance and toughness in the base material and also at the weld by decreasing the content of Al, so far admixed mainly for the purpose of deoxidation, to a minimum and adding Ti or Zr thereto as a deoxidizing element,-as in Japanese Patent Application Kokai (Laid-Open) No. 61-124554 corresponding to U.S. patent application Ser. No. 799978 and Japanese Patent Application Kokai (LaidOpen) No. 62-170458.
The Japanese Patent Application Kokai (Laid-Open) No. 61-124554 discloses the steel for production of a highly tough, ERW steel pipe having a distinguished sour resistance which is characterized by containing 0.01 to 0.35% by weight of C; 0.02 to 0.5% by weight of Si; 0.1 to 1.8% by weight of Mn; 0.0005 to 0.008% by weight of Ca; 0.006 to 0.2% in total by weight of at least one of Ti and Zr; not more than 0.005% by weight of Al; not more than 0.015% by weight of P; and not more than 0.003% by weight of S; or further containing (A) at least one of 0.2 to 0.6% by weight of Cu, 0.1 to 1.0% by weight of Ni, and 0.2 to 3.0% by weight of Cr, and/or (B) at least one of 0.10 to 1.0% by weight of Mo, 0.01 to 0.15% by weight of Nb and 0.01 to 0.15% by weight of V; the balance being Fe and impurities. Thus, the Al content of the steel is not more than 0.005% by weight.
The Japanese Patent Application Kokai (Laid-Open) No. 62-170458 was laid-open to the public on July 27, 1987 and discloses a steel for production of a highly tough, ERW steel pipe having a distinguished sour resistance which is characterized by containing 0.01 to 0.35% by weight of C; 0.02 to 0.5% by weight of Si; 0.1 to 1.8% by weight of Mn; more than 0.005% to 0.05% by weight of Al; 0.0005 to 0.008% by weight of Ca; 0.01 to 0.2% in total by weight of at least one of Ti and Zr; not more than 0.015% by weight of P; and not more than 0.003% by weight of S; Ti/Al, Zr/Al or (Ti+Zr)/Al being 2 or more by weight; or further containing (A) at least one of 0.2 to 0.6% by weight of Cu, 0.1 to 1.0% by weight of Ni, and 0.2 to 3.0% by weight of Cr, and/or (B) at least one of 0.10 to 1.0% by weight of Mo, 0.01 to 0.15% by weight of Nb and 0.01 to 0.15% by weight of V; the balance being Fe and impurities. And particularly the steel has Ti/Al, Zr/Al or (Ti+Zr)/Al of 2 or more by weight when the Al content is high, e.g. in a range of more than 0.005% to 0.05% by weight, and it contains Ti and Zr as main deoxidizing elements in place of Al to prevent formation of inclusions susceptible to deformation during the seam welding.